Method and plant for manufacturing cement clinker

ABSTRACT

A method and plant for manufacturing cement clinker are disclosed, in which cement raw meal is calcined in a calciner. The calciner comprises an upper zone and a lower zone, where fuel, combustion gas and raw meal are introduced into the upper zone of the calciner and directed downward through the calciner. The combustion gas and raw meal are introduced tangentially into the calciner and subject to rotation follow a spiral-shaped flow path, and, under the action of gravity, the raw meal is primarily directed down along the wall of the calciner. The rotation of the combustion gas in at least the lower zone of the calciner is subjected to braking and approximately simultaneously or subsequently at least some of the combustion gas and at least some of the raw meal in the lower zone of the calciner is directed in the direction towards the center-line of the calciner.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority from Denmark Patent No. 2002 00345filed on Mar. 7, 2002 and from PCT application number PCT/EP03/01164filed on Feb. 6, 2003, the disclosures of which are incorporated hereinby reference.

BACKGROUND OF THE INVENTION

The present invention relates to a method for manufacturing cementclinker by which method cement raw meal is calcined in a calcinercomprising an upper zone and a lower zone, where fuel, combustion gasand raw meal are introduced into the upper zone of the calciner anddirected downward through the calciner, where the combustion gas and rawmeal are introduced tangentially into the calciner and subject torotation follow a spiral-shaped flow path, and where, under the actionof gravity, the raw meal is primarily directed down along the wall ofthe calciner. The invention also relates to a plant for carrying out themethod.

A plant of the above-mentioned kind for manufacturing cement is wellknown from the literature.

A kiln plant of the Separate Line Calcinator Downdraft type (SLC-D) isknown for example from the U.S. Pat. No. 4,014,641 and WO 97/30003. Thistype of plant comprises a calciner, which is configured as a chamberwhich at its lower end is connected to an upward directed gas duct whichis also connected to the outlet end of the kiln for exhaust gases. Alsoknown from international patent application No. PCT/IB01/01296 is a kilnplant of the In Line Calcinator Downdraft type (ILC-D) where thecalciner consists in the main of one big chamber. It is a common featurefor the two types of plant that most of the fuel used in the calciner isinjected axially at the top of the calciner. Preheated air from theclinker cooler and preheated raw meal are introduced tangentially intothe top of the calciner. In the kiln plant of the type ILC-D, theexhaust gases from the kiln are also introduced tangentially into thetop of the calciner, typically at a location above the point at whichthe preheated air is introduced in order to establish a NO_(x)-reducingzone at the uppermost part of the calciner. In both types of plant thefuel is thus burned primarily in a zone around the centre-line of thecalciner, while, due to the tangential method of introduction, the rawmeal and combustion air will flow downward through the calciner along aspiral-shaped path, thereby thrusting the raw meal towards the calcinerwall, causing it to slide down along the wall under the action ofgravity. As a consequence hereof, simultaneously with the calcinationprocess, the raw meal will effectively protect the lining in thecalciner. Advantages of these known types of plant are that fuels with alow content of volatile constituents can be used and that formation ofNO_(x) is at a relatively low level. However, it is a distinctdisadvantage of such kiln plants that optimum calcination of the rawmeal discharged from the calciner has not been achieved. Also, theexhaust gases exiting the calciner may contain unburned fuel, of which amajor part will be in the form of CO, which will be ascribable to thefact that the combustion air is not mixed with the fuel to the extentrequired to provide the necessary air surplus across the entirecross-sectional area of the calciner.

SUMMARY OF THE INVENTION

It is the objective of the present invention to provide a method as wellas a plant for manufacturing cement by means of which it will bepossible to achieve a high calcination efficiency of the raw meal aswell as a high fuel combustion efficiency in the calciner.

This is obtained by a method of the kind mentioned in the introductionand being characterized in that the rotation of the combustion gas in atleast the lower zone of the calciner is subjected to braking and in thatapproximately simultaneously or subsequently at least some of thecombustion gas and at least some of the raw meal in the lower zone ofthe calciner is directed in the direction towards the centre-line of thecalciner.

Hereby is obtained an improvement in the calcination efficiency of theraw meal and the fuel combustion efficiency in the calciner. This is dueto the fact that the raw meal and the oxygen-containing air beingclosest to the calciner wall will thus be more effectively mixed withthe hotter exhaust gases and the fuel being in the central zone of thecalciner, which means that the raw meal will absorb a greater amount ofheat from the exhaust gases and the fuel will burn out when brought intocontact with the oxygen-containing air.

The plant for carrying out the method according to the invention is ofthe kind which comprises a substantially rotation-symmetrical calcinerwith an upper zone and a lower zone, which calciner comprises means forintroducing fuel into the upper zone of the calciner and means fortangential introduction of combustion gas and raw meal into the upperzone of the calciner, and being characterized in that it comprises meansfor braking the rotation of the combustion gas at least in the lowerzone of the calciner, and means for approximately simultaneous orsubsequent diversion of at least some of the combustion gas and at leastsome of the raw meal in the lower zone of the calciner in directiontowards the centre-line of the calciner.

In a preferred embodiment the means comprise a number of internalfittings formed with vertical side faces and an inclined top face. Thevertical side faces as well as the inclined top faces are substantiallyplane in order to facilitate any cleaning thereof from the outside. Theinternal fittings may be formed as right-angled circle sections. In aparticularly preferred embodiment the plant comprises four internalfittings constituting between them a cross-shaped free space.

Additional characteristics of the present invention will appear from thefollowing detailed description and the drawing.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be explained in further details in the following withreference being made to the drawing where:

FIG. 1 shows a plant of the type ILC-D for carrying out the methodaccording to the invention,

FIG. 2 shows a plant of the type SLC-D for carrying out the methodaccording to the invention,

FIG. 3 shows a cross-section through a preferred embodiment of thecalciner for carrying out the method according to the invention, and

FIG. 4 shows a longitudinal section along the line A—A through a sectionof the calciner shown in FIG. 3.

DETAILED DESCRIPTION OF THE INVENTION

In FIG. 1 is shown a kiln plant of the type ILC-D for manufacturingcement clinker. This plant comprises a cyclone preheater, whereof onlythe last cyclone 1 is shown, a calciner 3 with separation cyclone 4; arotary kiln 5, and a clinker cooler 7. The plant also comprises a kilnriser duct 9 for routing kiln exhaust gases to the calciner 3, and aduct 11 for directing preheated air from the clinker cooler 7 to thecalciner 3. Preheated raw meal is separated from the preheater in thecyclone 1 and directed to the calciner 3 in which it is calcined.Thereafter, from the bottom outlet of the separation cyclone 4, thecalcined raw meal is led via a duct 8 through to the rotary kiln 5 inwhich it is burned into cement clinker which is subsequently cooled inthe clinker cooler 7. The exhaust gases from the rotary kiln 5 and thecalciner 3 are drawn from the calciner 3 through the cyclone 4 andupstream through the preheater by means of a schematically illustratedfan 6.

In the shown embodiment fuel is introduced into the calciner 3 via aburner 13 which is arranged so that fuel is introduced axially at thetop of the calciner 3, while the exhaust gases from the rotary kiln 5are introduced via an inlet duct 15 which discharges tangentially intothe upper zone 3 a of the calciner. Therefore, the combustion of thefuel introduced into the calciner 3 via the burner 13 initially occursin an atmosphere consisting of kiln exhaust gases with a low oxygencontent. This will result in the generation of a reducing zone in whichthe NO_(x), which is supplied together with the kiln exhaust gasesreacts with the fuel, thereby reducing the NO_(x) level.

At some point further downstream on the calciner, after a reducing zoneof appropriate length, preheated air is introduced from the clinkercooler 7 via the duct 11 which discharges tangentially into the calciner3. The main part of the preheated raw meal from the preheater isintroduced into the calciner 3 as indicated at the arrow together withthe air from the cooler. Because of the tangential method ofintroduction the cooling air/raw meal suspension will flow downwardthrough the subsequent burn-out zone following a spiral-shaped path,whereby the raw meal will be slung out towards the wall of the calciner,where after it slides down along the calciner wall under the action ofgravity, while the air from the clinker cooler will envelop the flame inthe central part of the calciner, so that it is gradually mixed with theexhaust gas/fuel suspension. As a result, the amount of raw meal in thecentral part of the burn-out zone of the calciner will be quite small,and this means that a high temperature, and thus a high degree of fuelburn-out, can be attained even when using fuels having a low content ofvolatile constituents. The raw meal, which slides down along thecalciner wall, is calcined by accumulation of the heat from the centralburn-out zone of the calciner, and it thus serves as a heat shieldprotecting the calciner wall against the high temperatures, whichprevail in the burn-out zone.

In the lower zone 3 b of the calciner the raw meal is suspended in thedownward-directed stream of exhaust gases, leading to furthercalcination of the raw meal and a lowering of the exhaust gastemperature. The exhaust gas/raw meal suspension is subsequentlydirected via a transition section 3 c to the separation cyclone 4 inwhich the raw meal is separated from the exhaust gases and routed to therotary kiln 5 via the bottom outlet of the cyclone 4.

For controlling the temperature in the burn-out zone and to ensure asimultaneous reduction of the discharge temperature of the exhaust gasesfrom the calciner, a smaller amount of preheated raw meal may beintroduced into the lower zone 3 b of the calciner or into thetransition section 3 c, shown at the dotted arrow. A small amount ofpreheated raw meal from the preheater may further be introduced into thekiln exhaust gas stream immediately after the latter has been dischargedfrom the rotary kiln 5 as shown at the dotted arrow. As a result, thekiln exhaust gas temperature will be lowered, thereby reducing potentialcaking problems in the duct 9. Because of its catalytic effect, this rawmeal will also cause a further reduction of the NO_(x) level in thecalciner 3.

In FIG. 2 is shown a kiln plant of the type SLC-D for manufacturingcement clinker. This plant also comprises a cyclone whereof only thelast cyclone 1 is shown, a calciner 23 with an upper zone 23 a and alower zone 23 b, a rotary kiln 5, and a clinker cooler 7.

The plant also comprises a duct 11 for carrying preheated air from theclinker cooler 7 to the calciner 23. Preheated raw meal is separatedfrom the preheater in the cyclone 1 and routed together with the airfrom the clinker cooler 7 to the calciner 23 in which it is calcined.The suspension of raw meal and gas is then introduced into an upwardlydirected gas duct 29 in which it is mixed with the exhaust gases fromthe rotary kiln 5. The mixed suspension is then routed to a separationcyclone 4 from where the calcined raw meal is directed via a duct 8 tothe rotary kiln 5 in which it is burned into cement clinker which issubsequently cooled in the clinker cooler 7. The exhaust gases from therotary kiln 5 and the calciner 23 are drawn through the cyclone 4 andthen up through the preheater by means of a schematically shown fan 6.

In the example shown fuel is introduced into the upper zone 23 a of thecalciner 23 via a burner 13 which is fitted so that fuel is introducedaxially into the top of the calciner 23 while preheated air from theclinker cooler 7 is introduced via the duct 11 which dischargestangentially into the upper zone 23 a of the calciner 23. The main partof the preheated raw meal from the preheater is introduced into thecalciner 23 as shown at the arrow together with the air from the cooler.As is the case for the embodiment shown in FIG. 1, the suspension ofcooling air and raw meal will, because of the tangential mode ofintroduction, flow downward through the calciner 23 following aspiral-shaped path, whereby the raw meal will be slung out towards thewall of the calciner, where after it slides down along the calciner wallunder the action of gravity, while the air from the clinker cooler willenvelop the flame in the central part of the calciner, so that it isgradually mixed with the fuel. As a result, the amount of raw meal inthe central part of the burn-out zone of the calciner will be quitesmall, and this means that a high temperature, and thus a high degree offuel burn-out, can be attained. The raw meal, which slides down alongthe calciner wall, is calcined by accumulation of the heat from thecentral burn-out zone of the calciner, and it thus serves as a heatshield protecting the calciner wall against the high temperatures, whichprevail in the burn-out zone.

From the lower zone 23 b of the calciner 23, the suspension of calcinedraw meal and gas is subsequently introduced into the upward-directed gasduct 29 in which it is mixed with the exhaust gases from the rotary kiln5. In the gas duct 29 calcination of any not completely calcined rawmeal may be completed and fuel may also be introduced via a not shownburner. The mixed suspension is then routed as previously noted to theseparation cyclone 4 from where the calcined raw meal is directed to therotary kiln 5 via the duct 8.

The plants shown in FIGS. 1 and 2 each include a mixing unit 10 forimproving the level of calcination of the raw meal as well as theburn-out efficiency of the fuel in the calciner 3, 23. This mixing unit10 is installed in the lower zone 3 b, 23 b of the calciner 3, 23 in thearea schematically designated by means of a dotted circle in FIGS. 1 and2. The unit 10 is formed so that their function is primarily to brakethe rotation of the exhaust gases in the lower zone 3 b, 23 b of thecalciner 3 and secondarily to, simultaneously or subsequently, directthe combustion gas and raw meal towards the centreline of the calciner3, 23. The result will be a much improved mixture between the raw mealand the oxygen-containing airstream concentrated nearest the calcinerwall, and the hotter exhaust gases and fuel in the central zone of thecalciner, enabling the raw meal to absorb a greater amount of heat fromthe exhaust gases and ensuring a higher fuel burn-out rate on contactwith the oxygen-containing air.

FIGS. 3 and 4 show a preferred embodiment of the mixing unit 10installed in the calciner 3, 23 according to the invention. In FIG. 3the mixing unit 10 is seen to comprise four internal fittings 31identically formed as right-angled circle sections and constitutingbetween them a free space having the form of a cross. The internalfittings are, as is best illustrated in FIG. 4, formed with verticalside faces 33 and an inclined top face 35. The fittings 31 are made ofmetal that is protected by an inner lining. The side faces 33 willeffectively brake the rotation of the gas stream while the top faces 35will convey the gas as well as raw meal in the direction of thecentre-line of the calciner, thereby forcibly ensuring greater mixingefficiency.

1. A method for manufacturing cement clinker by which method cement rawmeal is calcined in a calciner comprising an upper zone and a lowerzone, where fuel, combustion gas and raw meal are introduced into theupper zone of the calciner and directed downward through the calciner,where the combustion gas and raw meal are introduced tangentially intothe calciner and subject to rotation to follow a spiral-shaped flowpath, and where, under the action of gravity, the raw meal is primarilydirected down along the wall of the calciner, wherein the rotation ofthe combustion gas in at least the lower zone of the calciner issubjected to braking and in that approximately simultaneously orsubsequently at least some of the combustion gas and at least some ofthe raw meal in the lower zone of the calciner is directed in thedirection towards the centre-line of the calciner.
 2. A plant formanufacturing cement clinker comprising a substantiallyrotation-symmetrical calciner having an upper zone and a lower zone,means for introducing fuel into the upper zone of the calciner and meansfor tangential introduction of combustion gas and raw meal into theupper zone of the calciner, and a mixing unit for braking the rotationof the combustion gas at least in the lower zone of the calciner andapproximately simultaneously or subsequently diverting at least some ofthe combustion gas and at least some of the raw meal in the lower zoneof the calciner in the direction towards the centre-line of thecalciner.
 3. A plant according to claim 2, wherein the mixing unitcomprises a number of internal fittings formed with vertical side facesand inclined top face.
 4. A plant according to claim 3, wherein thevertical side faces and the inclined top faces are planar.
 5. A plantaccording to claim 3, wherein the internal fittings are formed asright-angled circle sections.
 6. A plant according to claim 3, whereinthe internal fittings form a cross-shaped free space there between.